This invention relates to electrical cable and conduit. More particularly, it relates to a system which retards lightning so that the cable or conduit is not substantially affected by the lightning and, in the case of communication cable, the communication signal on a signal conductor within the cable is not substantially affected, as well as its associated equipment.
While this invention is applicable to both power and communication cable, most of the detailed discussion herein will focus on communication cable used in conjunction with an antenna.
As used herein, the term antenna includes television and radio antenna, satellite dishes and other devices which receive electromagnetic signals. A major problem associated with an antenna is caused by lightning striking the antenna. Often the high current associated with the lightning will travel through the communication cable which is attached between the antenna and electronic equipment. This current will damage the electronic equipment.
According to The Lightning Book, by Peter E. Viemeister, self-induction in a conductor may occur during a lightning strike. This occurs because lightning currents may rise at a rate of about 15,000 amperes in a millionth of a second. For a straight conductor with the usual cross section, this surging current can produce nearly 6,000 volts per foot of wire, which is enough to jump an insulated gap to a nearby conductor, such as the center conductor, in a coaxial cable.
Currently lightning protection of cable is more focused on the installation of cable within a system. The National Electric Code attempts to insure a proper path for lightning to discharge, thus reducing the damage of equipment connected to the end of the cable. The cable in and of itself offers little or no protection from electric fields or magnetic fields associated with the lightning strike. Even though electrical codes provide suggestions on installing and grounding equipment, their primary focus is providing a straight path to ground for lightning to discharge and eliminating the differences of potential between the two items.
FIG. 1 is an example of a home TV antenna installation according to the National Electric Code. If lightning were to strike antenna 10, half of the charge would be on ground wire 12 which is attached to the mast 14 of the antenna, and the other half would be on the coaxial cable""s outer shield 16 which is connected to the antenna terminals 18. Theoretically, the current on coaxial cable 16 would travel to antenna discharging unit 20 and then through grounding conductor 22. The center conductor or signal conductor of the coaxial cable, however, is unprotected, which means that damage to the electronics in the receiver and other components within the home is likely. Furthermore, the longer the lead-in wire, the greater the problem. As lightning strikes this antenna 10 and discharges to ground, a large electric field is set up along the coaxial lead-in wire 16 and ground wire 12. At right angles to this electric field is an exceptionally strong magnetic field which surrounds all of the cable.
In addition, lightning follows the straightest, closest and best path to ground. Any sharp bends, twists or turns of the ground wire sets up resistance to the quick discharge. See Page 201 of The Lightning Book, referred to above. This resistance usually causes the discharge to jump off the ground wire with the bend and into a path of least resistance.
It is one object of this invention to provide an improved lightning retardant cable which may or may not be received in a conduit.
It is another object to provide a lightning retardant cable which deals with both electric and magnetic fields caused by lightning.
It is also an object to provide a system for use with cable, conduit and/or pipes which retards lightning.
In accordance with one form of this invention there is provided a lightning retardant cable which includes at least one internal conductor. The internal conductor may be a signal conductor or a power conductor. A signal conductor conducts a signal containing information. A power conductor conducts current for operating devices and equipment.
A choke conductor is provided. The choke conductor is wound about the internal conductor in the shape of a spiral. The choke conductor is not in contact with the internal conductor. The choke conductor presents a high impedance to the electrical current caused by lightning when the lightning strikes near the cable.
Preferably, the internal conductor is made of metal for conducting electrical signals or current, although the internal conductor may be an optical fiber.
It is also preferred that a spiraled shield be placed underneath the choke conductor. The spiraled shield is also wound about the internal conductor, but in an opposite direction to the choke conductor. The adjacent windings of the shield are not in electrical contact with one another and act as another choke. Preferably, 90xc2x0 angles are formed at the crossing points between the choke conductor and the shield.
The choke conductor dissipates the electric field caused by the lightning strike. The shield performs two functions. It acts as a choke in the opposite direction of the choke conductor and thus enhancing the cancellation process.
It is also preferred that one side of the shield be insulated so that when the shield is wound about the cable a winding is not in electrical contact with the previous or next winding. The insulation under the shield may extend over one of the edges of the shield to reduce the likelihood of arcing.
The choke conductor may also be insulated. The choke conductor may be substantially rectangular in shape with, preferably, round edges. In addition, each end of the insulated choke conductor may be electrically connected to a corresponding end of the shield. This connection may be made by winding an insulated part of the choke conductor about an uninsulated part of the shield at each end of the cable.
It is also preferred that an overall outer jacket be provided for the cable and that a ground conductor be attached to the outer jacket.
Also, the choke conductor and shield may be wound about the cable as described above, or they may be wound about a conduit which receives the cable. It is preferred that the induction of the choke and the shield be substantially equal. The number of turns in which the choke is wound may be adjusted to equalize their inductance.
In another form of this invention, there is provided a lightning retardant system including a pipe. The pipe has an outer surface. At least one insulated electrical conductor is located adjacent to the outer surface. A shield is spiraled about the outer surface in one direction. A choke conductor is adjacent to the shield and spiraled about the outer surface in a direction opposite to the shield.
In another form of this invention, there is provided a lightning retardant cable including at least one internal conductor. A jacket covers the conductor. A flat metal shield having first and second longitudinal edges is provided. A layer of insulation covers one side of the shield and extends beyond at least one of the longitudinal edges, thereby forming an insulated shield. The insulated shield is spiraled about the jacket.
In another form of this invention, there is provided a lightning retardant system including a cable having at least one internal conductor. A jacket covers the conductor. A flat metal shield is provided having first and second longitudinal edges. A layer of insulation covers one side of the shield and extends beyond at least one of the longitudinal edges, thereby forming an insulated shield. The insulated shield is spiraled about the jacket. One end of the conductor is connected to a signal receiver and the other end of the conductor is connected to a signal decoder. The signal receiver is connected to ground. The shield is also connected to ground at a place remote from the signal receiver.
In another form of this invention, there is provided a lightning retardant cable assembly including at least one conductor. A flexible conduit surround the conductor. The flexible conduit is spiraled and has overlapping edges. An amount of electrical insulation is located between the overlapping edges so that the conduit will perform as a choke presenting high impedance to lightning.